I think I've figured it out. I believe that the server stores the length of your mog locker lease as seconds since midnight on January 1st, 2002.
Timestamp for above date: 3,157,173,167
Maximum value of a signed 32-bit integer: 2,147,483,647
The difference: 1,009,689,520
That timestamp as a date: Sun, 30 Dec 2001 05:18:40
There's a +/- 5 day uncertainty because that's the increment in which you add time, but the numbers are just so very coincidentally close in agreement I can't imagine it's done any other way.